Viola Jones Algorithm
An introduction to the Haar features is provided in the youTube video.  The video indicates the VJ face detector leverages a selected combination of Haar features (convolutional kernels) to detect facial features (weak classifiers), such as the nose bridge.  The binary presence of the weak classifiers are summed to determine if the window contains a face.

The ability for a VJ algorithm to detect emotion would rely on the ability to assign a set of Haar features (kernels) to recognize features associated with a particular emotion label (surprise, anger, content, fear). 

It is conceivable that the initial stage of an  emotion classifier could use a the VJ algorithm to identify a face for additional stages to classify emotion.